The numbers on the speed graphic seems not to change when I twist the knob
and if I do twist the knob than it unwinds itself

I see in your profile you have a Anniversary Speed so I assume you're asking about the tension knob.
The military version is the C3. Download this C3 manual
https://www.cameramanuals.org/prof_pdf/c-3_graflex_camera.pdf
The snail may not be tightened against the tension knob properly. If the knob is turned counter clockwise without loosening the snail the tension shaft will twist off at the knob end ruining the shaft.

when I bought the camera it was missing the snail, Is there any place where I can buy the replacement

The initial tension is not set and will not be held without the snail.
The only source is a parts body or the thin chance a NOS part will show up on the auction site. That shutter design was used from 1928 to 1946 and may have been used from 1907 to 1946.

so I cannot wind the shutter with out the snail lock

1st. set the tension then using the wind key wind to the desired opening/speed.
Without tension the shutter will not run. You can wind the shutter to a opening but it will not trip/run down when the release is pressed.

what is the wind key

http://www.southbristolviews.com/pics/Graphic/manual-pdf/Anniversary.pdf
download and READ!